This invention relates to testing and characterization of low noise microwave and RF transistors and amplifiers (device under test, DUT); the method disclosed comprises a test setup for data acquisition and processing for extracting the “four noise parameters” of said DUT. The test setup uses automatic microwave tuners in order to synthesize reflection factors (or impedances) at the input of said DUT and allow collecting the necessary data using appropriate high sensitivity receivers.
All RF two-ports using semiconductor devices (DUT) contain internal noise sources which affect the purity of the signal entering at the input port and exiting (amplified) at the output port. A common way of characterizing the “purity” of the DUT at each frequency and bias condition is the noise figure: F. One definition of the noise figure is the degradation of the signal to noise ratio (S/N) between the input and output port of the DUT: F=(S.in/N.in)/(S.out/N.out) (equation 1), whereby S.in and S.out are the signal power levels at the input and output of the DUT and N.in and N.out the corresponding noise power levels. Since the DUT adds to the transmitted signal its internal noise, the S/N ratio at the input is higher than at the output, therefore F>1.
It has been established (see reference 1) that four real numbers fully describe the noise behavior of any linear noisy two-port; these are the four noise parameters. By generally accepted convention the four noise parameters (4NP) are: Minimum Noise Figure (Fmin), Equivalent Noise Resistance (Rn) and Optimum Noise Admittance (Yopt=Gopt+j*Bopt) (see reference 1). The noise behavior of a two-port only depends on the admittance of the source and not of the load. The general relationship is: F(Ys)=Fmin+Rn/Re(Ys)*|Ys−Yopt|2 (equation 2).
F(Ys) in equation 2 being the noise figure of the chain including the DUT and the receiver, the natural law of cascaded noisy two-ports described by FRIIS (see reference 2) is used to extract the noise figure of the DUT itself: FRIIS' formula is: F.dut=F.total−(F.rec−1)/Gav.dut(Sij) (equation 3); hereby F.dut is the noise figure of the DUT, F.rec is the noise figure of the receiver and Gav.dut is the available Gain of the DUT for the given frequency and bias conditions. Whereas F.total can be measured directly (see reference 6) F.rec and Gav.dut depend both, (a) on the small signal properties of the DUT, which are customarily described using the s-parameters, and (b) on the source admittance Ys per equation 2 and reference 3; Sij are the DUT S-parameters (see reference 3).
During calibrations needed in order to extract the receiver noise figure in equation 3 and during measurements, when the signal flow is switched between noise path (516) and signal path (52) in FIG. 2, any mechanical switching repeatability error of the switches (53) and (510) will affect the result. This happens because the mechanical RF switches are part of the error-correction two-port, also called “error-box” (FIGS. 7 and 13); in that case, even good switches may create an error. This is due to the complex mathematical operations required in error-term component calculations (FIG. 13), which may amplify even small changes in measurement topology (switching changes the RF behavior of the measurement path, FIG. 6).
Commonly used prior art test setups are shown in FIGS. 1 and 2: Referring to prior art FIG. 1 the test system comprises: a calibrated noise source (52), an impedance tuner (60), a test fixture (10) to hold the DUT and a sensitive noise receiver (72). The tuner (60) and the noise receiver (72) are controlled by a system computer (not shown), which sets the source admittance Ys, created by the tuner, and retrieves digitally the associated noise measurement data from the noise receiver (72). After termination of the measurement session the computer program processes the measured data and extracts the four noise parameters of the DUT for a given frequency and DUT bias conditions. To measure DUT s-parameters the switches (54) and 64) are switched towards the network analyzer (70); to measure noise figure they are switched towards the noise source (52) and the noise receiver (72); it is this switching process which creates a measurement uncertainty and therefore a potential error, depending on the quality and repeatability of the switching process, since all calculations and corrections must assume that the switches are perfectly repeatable, which is rarely the case.
From equation 2 follows that, in order to determine the four noise parameters, one would have to take four measurements at four different source admittance values Ys. However, noise measurements are extremely sensitive and various disturbances cause measurement errors and uncertainties. It is therefore the accepted procedure to acquire more than four data points, at each frequency and extract the noise parameters using a linearization and error minimization technique. This method has been used and refined for many years (see reference 5 and FIGS. 1, 2 and reference 6) but is in general cumbersome and prone to insufficiencies, since the DUT may oscillate or the impedance tuner itself may create measurement errors, which are difficult to identify and eliminate if there are not enough data points to extract the four noise parameters from. The conclusion is that, to improve the reliability of the measurement one needs more data and elaborated extraction algorithms in order to deal with the noise parameter extraction problem as a statistical observation event.
In equation 3 the available gain of the DUT can only be calculated using the DUT s-parameters; these s-parameters must be accurate and measured, if possible immediately before the noise data acquisition, to avoid device drifting and allow calculations using equation 3. This is the reason for using RF switches in the measurement path (see reference 4 and FIGS. 1, 2). These switches allow fast toggling between the noise measurement path (516) and the s-parameter (signal) path (52) in FIG. 2. The problem is that those switches (see FIGS. 3 and 4) are not perfect. Their repeatability varies from unit to unit, deteriorates with increasing frequency and is limited (see FIG. 7); it may therefore create significant measurement errors and dispersion, both in DUT s-parameter measurement and noise measurement. This is due to the internal mechanics of the switches, as shown in FIGS. 5 and 6. As is shown in FIGS. 7 and 14 the repeatability of the switches itself is a potential source of error and affects the end result after calibrating the system.
This invention discloses a test setup that performs the same tasks as the traditional setup without using RF switches and the thereby associated un-correctable repeatability error.